The recovery of hydrocarbons from subterranean zones relies on the process of drilling wellbores. This process includes drilling equipment situated at the surface and a drill string extending from the surface equipment to the formation or subterranean zone of interest. The drill string can extend thousands of feet or meters below the surface. The terminal end of the drill string includes a drill bit for drilling, or extending, the wellbore. The process also relies on some sort of drilling fluid system, in most cases a drilling “mud”. The mud is pumped through the inside of the drill string, which cools and lubricates the drill bit and then exits out of the drill bit and carries rock cuttings back to surface. The mud also helps control bottom hole pressure and prevents hydrocarbon influx from the formation into the wellbore and potential blow out at the surface.
Directional drilling is the process of steering a well from vertical to intersect a target endpoint or to follow a prescribed path. At the terminal end of the drill string is a bottom hole assembly (BHA) which may include 1) the drill bit; 2) a steerable downhole mud motor of a rotary steerable system; 3) sensors of survey equipment for logging while drilling (LWD) and/or measurement while drilling (MWD) to evaluate downhole conditions as drilling progresses; 4) apparatus for telemetry of data to the surface; and 5) other control equipment such as stabilizers or heavy weight drill collars. The BHA is conveyed into the wellbore by a string of metallic tubulars known as the drill string. MWD equipment may be used to provide downhole sensor and status information at the surface while drilling in a near real-time mode. This information is used by the rig crew to make decisions about controlling and steering the well to optimize the drilling speed and trajectory based on numerous factors, including lease boundaries, existing wells, formation properties, hydrocarbon size and location. These decisions can include making intentional deviations from the planned wellbore path as necessary, based on the information gathered from the downhole sensors during the drilling process. In its ability to obtain real time data, MWD allows for a relatively more economical and efficient drilling operation.
Various telemetry methods may be used to send data from MWD or LWD sensors back to the surface. Such telemetry methods include, but are not limited to, the use of hardwired drill pipe, acoustic telemetry, use of fibre optic cable, mud pulse (MP) telemetry and electromagnetic (EM) telemetry.
EM telemetry involves the generation of electromagnetic waves at the wellbore which travel through the earth's surrounding formations and are detected at the surface.
Advantages of EM telemetry relative to MP telemetry, include generally faster baud rates, increased reliability due to no moving downhole parts, high resistance to lost circulating material (LCM) use, and suitability for air/underbalanced drilling. An EM system can transmit data without a continuous fluid column; hence it is useful when there is no mud flowing. This is advantageous when the drill crew is adding a new section of drill pipe as the EM signal can transmit the directional survey while the drill crew is adding the new pipe.
Disadvantages of EM telemetry include lower depth capability, incompatibility with some formations (for example, high salt formations and formations of high resistivity contrast), and some market resistance due to acceptance of older established methods. Also, as EM transmission is strongly attenuated over long distances through the earth formations, it requires a relatively large amount of power so that the signals are detected at surface. Higher frequency signals attenuate faster than low frequency signals.
A BHA metallic tubular is generally used as the dipole antennae for an EM telemetry tool by dividing the drill string into two conductive sections by an insulating joint or connector which is known in the art as a “gap sub”. A voltage is driven between the two conductive sections to produce an electromagnetic signal.
A gap sub must withstand the mechanical loading induced during drilling and the high differential pressures that occur between the center and exterior of the drill pipe. These mechanical loads are typically quite high and most drill string components are made from high strength, ductile metal alloys in order to handle the loading without failure. As most high dielectric materials typically used in gap sub assemblies are either significantly lower strength than metal alloys or highly brittle, the mechanical strength of the gap sub becomes a significant design hurdle. The gap sub tends to be a weaker link in the drill string.